1. Field of the Invention
The invention relates to a method of obtaining a structure on a semiconductor wafer by etching through structures defined by an etch mask using a plasma.
2. Description of the Related Art
In semiconductor plasma etching applications, a plasma etcher is usually used to transfer a photoresist mask pattern into a circuit and line pattern of a desired thin film and/or filmstack (conductors or dielectric insulators) on a Si wafer. This is achieved by etching away the films (and filmstacks) underneath the photoresist materials in the opened areas of the mask pattern. This etching reaction is initiated by the chemically active species and electrically charged particles (ions) generated by exciting an electric discharge in a reactant mixture contained in a vacuum enclosure also referred to as a reactor chamber. Additionally, the ions are also accelerated towards the wafer materials through an electric field created between the gas mixture and the wafer materials, generating a directional removal of the etching materials along the direction of the ion trajectory in a manner referred to as anisotropic etching. At the finish of the etching sequence, the masking materials are removed by stripping it away, leaving in its place replica of the lateral pattern of the original intended mask patterns. This etching method is illustrated in FIGS. 1A-C. In this method, a plasma etching process is used to transfer directly the photoresist mask pattern 104 into that of the underlying oxide dielectric thin film 108, as shown in FIG. 1A. The etching generates a contact hole 112 and erodes and damages the, photoresist 104, as shown in FIG. 1B. The photoresist is then removed leaving the contact hole 112 in the oxide 108, as shown in FIG. 1C. During the etching process, the mask materials are usually eroded and/or damaged in exchange for the pattern transfer. Consequently, some of the damage and erosion also may be transferred to the underlying layers leaving such undesirable pattern distortions such as striation, CD enlargement, etc.
The objective of the etching methodology, therefore, includes reducing the photoresist mask erosion to enhance the fidelity of the pattern transfer from the photoresist mask patterns. For this purpose, it has been proposed to include a passivation gas in the reactive etching mixture. This passivation gas can be chosen in such a way that its presence selectively reduces the etching damage and erosion of the masking materials relative to the removal rate of the thin film materials to be etched. The passivation gas can be chosen in such a way that, an etching retardation coating is generated on the surface of the masking materials acting as a barrier to slow down the etching reaction. By design, the passivation gas is chosen in a way that it additionally beneficially forms an etching retardation coating on vertical surfaces of the film structures to be etched, such that etching reaction can not advance in the absence of the ion bombardment. By the nature of the vertical trajectory of the charged particles, etching can therefore advance only in the vertical direction, with little to no etching in the lateral direction, creating an anisotropic etching profile. Hence, the presence of a passivation gas in the etching mixture is very important for the advantage of better etching mask protection and highly anisotropic etching profile by the use of relatively high energy directional ion bombardment.
It has already been proposed that the reactive gas mixture contain etching gases and polymer formers, with the latter acting the role of a passivation gas. In this case, the etching gases release highly reactive species by the excitation of an electrical discharge, which in turn etches the thin film materials to be etched as well as the masking materials by the mechanism of a spontaneous reaction. By the nature of spontaneous reactions, the etching reaction advances in both the vertical as well as the lateral surfaces, creating isotropic etching profiles. The co-presence of a polymer former, through generation of a polymer deposit on the surface of the etching structures and masking materials, can be used to create simultaneously high etching selectivity to masking materials and etching anisotropy, in conjunction with the ion bombardment.
It also has already been proposed that the reactive gas mixture contain polymer former gases and an etching enabler gas. The role of the etching enabler gas is to enable the polymer former gas to release highly reactive species by reacting with the polymer former gases in the presence of an electrical discharge. Alternatively, a retardation coating on the etching materials as well as the masking materials can also be formed by chemical reaction of a properly chosen passivation gas directly with the surfaces of these materials.
A common disadvantage of the above mentioned methods is that the optimum conditions for different aspects of the etching requirement usually do not coincide and by mixing the gases some of the unique properties of each precursor gases may be lost due to inter-reactions. The etching condition optimization almost always involve complex trade-offs into a single etching condition that may not be the optimum should the different etching chemistries be separate.
A variant of the etching methodology is taught in U.S. Pat. No. 5,501,893, issued Mar. 26, 1996 to Laermer et al., entitled xe2x80x9cMethod of Anisotropically Etching Siliconxe2x80x9d. This method separates out the etching gases and polymer former gases into two different steps, each consisting purely of one type of chemicals but not the other. This allows for fast etching rate at low ion bombardment energies, since at low ion bombardment energies, high selectivities to masking materials can be achieved for certain spontaneous etching reactions if the activation energy is slightly lower for the reaction at the surface of the etching materials than the masking materials. By removing the polymer former from the etching process, on the other hand, the etching. process would necessarily be isotropic during the duration when the etching is proceeding, since there is no retardation layer to prevent the lateral etching from occurring. Additionally, without the passivation gas in the etching mixture, it would be difficult to obtain sufficient etching selectivity to the masking materials if the desire is there to use higher ion energies. Many etching applications can benefit from high ion bombardment energy to obtain high aspect ratio structures in very small dimension structures, for example.
Additional proposed methods include a stacked masking scheme to improve the overall etching resistance of the masking materials. This is illustrated in FIGS. 2A-F. In FIG. 2A an oxide layer 204 is provided. FIG. 2B shows a hardmask layer 208 placed over the oxide layer. A photoresist mask 212 is placed over the hardmask layer 208, as shown in FIG. 2C. The photoresist mask 212 is used to pattern the hardmask layer 208 to create a patterned hardmask layer 214, and the photoresist layer 212 may be removed, as shown in FIG. 2D. A contact hole 216 is etched in the oxide layer 204, using the patterned hardmask layer 214 as a mask as shown in FIG. 2E. The hardmask is then removed leaving the contact 216 in the oxide layer 204, as shown in FIG. 2F.
The advantages of this method are that, by having a more inert hardmask from which to transfer patterns (circuits and lines) to the underlying films, the etch performance is much enhanced and the requirement on the etching and photolithography is also much reduced. The disadvantages of this method are that, by introducing new process steps and new tool sets into the process flow, it is of higher cost and lower overall throughput. In addition, the extra process complexity also introduces difficulties by itself. For example, the Si hardmask used for dielectric contact etch applications is not as easily stripped as the photoresist mask.
The purpose of this invention is to provide a generic method for etching a feature in a layer or a stack of layers to obtain a high fidelity replica of a lateral pattern formed by a masking material with simultaneously high etching anisotropy and high selectivity to the masking materials as well as to the stop layers
To achieve the foregoing and in accordance with the purpose of the present invention, a method for etching a feature in a layer through an etching mask is provided. A protective layer is formed on exposed surfaces of the etching mask and vertical sidewalls of the feature with a passivation gas mixture. The feature is etched through the etching mask with reactive etching mixtures containing at least one etching chemical and at least one passivation chemical.
In another embodiment of the invention, an apparatus for etching a layer under an etch mask, where the layer is supported by a substrate, is provided. A plasma processing chamber comprising a chamber wall forming a plasma processing chamber enclosure, a substrate support for supporting a substrate within the plasma processing chamber enclosure, a pressure regulator for regulating the pressure in the plasma processing chamber enclosure, at least one electrode for providing power to the plasma processing chamber enclosure for sustaining a plasma, a gas inlet for providing gas into the plasma processing chamber enclosure, and a gas outlet for exhausting gas from the plasma processing chamber enclosure is provided. A deposition gas source and an etchant gas source are provided. A first control valve in fluid connection between the gas inlet of the plasma processing chamber and the deposition gas source and a second control valve in fluid connection between the gas inlet of the plasma processing chamber and the etchant gas source are provided. A controller controllably connected to the first control valve, the second control valve, and the at least one electrode comprising at least one processor and computer readable media is provided. The computer readable media comprises computer readable code for opening the first control valve for at least one deposition step to provide a deposition gas from the deposition gas source to the plasma processing chamber enclosure, computer readable code for closing the second control valve for the at least one deposition step to prevent etching gas from the etching gas source from entering the plasma processing chamber enclosure, computer readable code for opening the second control valve for at least one etching step to provide an etching gas from the etching gas source to the plasma processing chamber, and computer readable code energizing the at least one electrode to provide a bias of greater than 250 volts on the substrate for at least one etching step.
These and other features of the present invention will be described in more details below in the detailed description of the invention and in conjunction with the following figures.